[evanelias]

Facebook (along with almost everyone else using MySQL at massive scale) doesn't use foreign keys.

They scale poorly in MySQL, and they lose a lot of purpose in a massively sharded environment anyway. For example, say you like a status post on Facebook, or friend another user. It's very unlikely that the liked status or friended user exists on the same shard as your account, and there's no way to enforce a foreign key relationship in an inherently non-distributed database like MySQL.

So instead integrity is handled at the application layer, with additional background processes to fix the occasional integrity problem and detect integrity anomalies.